"If I get the opportunity, I'm going to make shots," said Cassell, who was playing in just his fourth game since signing with the Celtics March 4. "You know me, I'm going to make some shots. They gave me the opportunity to make the shot and I just made it."
The Celtics have won 12 of 13 games and six straight on the road. The Spurs have dropped four straight, and the loss snapped a seven-game home winning streak.
Three Celtics starters had at least 20 points in what may have been the biggest road win of the season: Paul Pierce (22), Kevin Garnett (21), and Rajon Rondo (20).
"That one made me sweat," said Celtics coach Doc Rivers, his forehead beading with perspiration after the game. "It was a great game."
The Celtics scored a season-low 11 points in the first quarter (they trailed, 28-11) and were down as many as 22 (35-13) before coming back to take a 56-51 lead in the third quarter. San Antonio entered the fourth quarter ahead just 70-68.
San Antonio seemed to be in good shape as two free throws by Manu Ginobili (game-high 32 points) pushed Boston behind, 81-72, with 5:56 remaining. But the Celtics responded with a 12-4 run finalized by an Eddie House 3-pointer to trim their deficit to 85-84 with 1:45 remaining.
"I'm really proud of our guys because San Antonio came out and they attacked us," Rivers said. "They went after us. We didn't respond to it very well. The lesson there is, on the road, you can't ease into a basketball game against an angry great team.
"That's what they were. They lost three in a row. You can see them, they had a purpose."
Said Spurs forward Robert Horry, "I'm mad that we were up 22 and blew it. I know we're playing one of the best teams in the league in the Celtics, but there is no excuse."
With Boston down, 87-86, in the final minute, the ball was swung around to Cassell at the top of the key. He nailed a 25-footer to push the Celtics ahead, then the 38-year-old skipped down the court and was mobbed by his new teammates as the Spurs called a timeout.
"That's what we brought him here for," Pierce said. "He kept us in the game, pretty much, the whole second quarter when we got down. And late in the game, that's what he does. He knocks down the big shot."
Garnett, who played with Cassell in Minnesota, said, "That's what it is. He was great for us."